GB2152252A - Data compression - Google Patents

Abstract

A stream of source characters, each having a time-varying frequency of appearance, is encoded into a stream of codewords. A rank store stores the rank corresponding to the current estimated relative frequency of appearance of each of the source characters in the stream. A rank updater exchanges the rank of each of the source characters which appear in the stream with the rank of the next lower ranking one of the source characters unless the source character in question has the lowest possible rank.

Description

1 GB 2 152 252A 1

SPECIFICATION

Data compression This invention relates to encoding a stream of source characters into a shorter stream of codewords and decoding the codewords.

Different source chdracters may appear with different frequencies in a stream of source characters. Encoding more frequently appear ing source characters as shorter codewords can reduce the length of the stream of co dewords. The relative frequencies of appear ance of different source characters can change over time.

According to the present invention, there is provided apparatus for encoding a stream of source characters into a stream of codewords, each of said source characters having a time varying frequency of appearance in said stream of source characters, said apparatus comprising: a rank store for storing the rank corresponding to the current estimated rela tive frequency of appearance of each said source character in said stream of source characters; and a rank updater for exchanging the rank of each said source character which appears in said stream of source characters with the rank of the next lower ranking one of said source characters, unless said source character which appears in said stream has the lowest possible rank.

In the apparatus described in detail below, the apparatus includes means for encoding according to a selectable code each of said 100 source characters having a rank not higher than a selectable first number as a codeword having one subword, each of said source characters having a rank higher than said first number and not higher than a second number 105 as a codeword having two subwords, and each of said source characters having a rank higher than said second number as a co deword having three subwords, said second number being fixed by the value of said first number.

The described apparatus also features a decoder for decoding the stream of codewords into a stream of source characters, the deco der comprising a decoder frequency ranker for determining for each of the source characters in the source alphabet the rank corresponding to its current estimated relative frequency of appearance. The decoder determines directly from the codewords in the stream of co dewords which one of the selectable codes was used for the encoding.

The reader should also refer to the specifi cation of British Patent Application number 8229520 (serial number 2109204) from which the present application has been div ided, and also to co-pending British Patent Application number 8502325 (serial number ) filed the same day as this applica tion and similarly divided from our aforesaid 130 application 8229520. The two applications referred to contain description common to the present application and relate to inventions which are also embodied in the apparatus described in detail herein below.

As we believe will become apparent from the description below, in our described apparatus, efficient, rapid, adaptive encoding of a source character stream into a compressed codeword stream is possible; the code changes to accommodate changes in the relative frequency of appearance of the source characters to minimize the length of the codeword stream; code adaptations may be made economically and quickly by changing only one free variable, from which the other relevant numbers can be recalculated easily; and the decoder can adjust to changing codes automatically based on the codewords re- ceived, without requiring special code-switching signals to be sent from the encoder to the decoder.

We turn now to the structure and operation of the preferred embodiment, first briefly des- cribing the drawings thereof.

Drawings Figure 1 is a block diagram of an encodingdecoding system. 95 Figure 2 is a table illustrating a typical coding sequence. Figure 3 is a block diagram of the encoder of Fig. 1. Figure 4 is a block diagram of the decoder of Fig. 1. Figure 5A through 5D are flow charts of the encoding process. Figure 6A through 6D are flow charts of the decoding process.

Overall Structure and Operation In Fig. 1, terminal 10 generates a stream of source characters on line 12 in the form of voltages, currents, frequencies or other para- meters representing binary digits. Encoder 14 receives the source characters at its port 16 and encodes them into a stream of binary codewords which are transmitted through port 18 onto link 20. The number of bits in the stream of codewords is generally smaller than the number of bits in the related stream of source characters. The form of the codewords permits independent decoding to obtain the original stream of source characters- Link 20 may perform a variety of functions, including modulating and demodulating, switching, storing, and controlling transmission errors, but in any case link 20 eventually delivers the same sequence of codewords to port 22 of decoder 24 as were received from encoder 14. Decoder 24 decodes the codewords into a stream of source characters identical to the stream received at port 16 and delivers it through port 26 to line 28, which transmits it to t,rminal 30.

2 GB 2 152 252A 2 Another stream of data may be flowing in the opposite direction through the system at the same time, with respect to which decoder 24 acts as an encoder and encoder 14 acts as a decoder.

Each source character is one of 256 possible 8-bit bytes. Each codeword is made up of 1, 2 or 3 subwords (called nibbles) of 4 bits each.

In general, the first step in the encoding process is to rank the 256 source characters in order of decreasing frequency of appearance in the source stream. Two tables are kept by the encoder, one being a list of ranks (from 0 to 255) in source character order in which the ith entry is the rank of the ith source character; the second table being a list of source characters (from 0 to 255) in rank order in which the ith entry is the source character of rank i. When a source character of rank r appears in the source stream, the two tables are updated so that the rank of that source character becomes r-1 (if it is not already zero), and the source character that had rank r-1 is given rank r. Accordingly the two tables are constantly updated to reflect the relative frequencies of appearance of the source characters. The rank (before the tables are updated) of each source character appear- ing in the source stream determines the codeword into which it will be encoded.

For purposes of encoding, the 256 possible source characters are divided into three groups. Source characters of ranks from zero to a first number (N4E) are encoded as onenibble codewords, those of ranks from N4E to a second number (N8E-1) are encoded as two-nibble codewords, and those of ranks from N8E to a third number (M) are encoded as three-nibbie codewords.

The determination of N4E, N8E and NI- is governed by the so-called Kraft equality:

N4E (N8E-N4E) NL-NElE -+ - + -= 1 NS N S2 N S3 where NS is the number of different available nibbles, and NI- is a selected number (here 267) at least as great as the number of possible source characters. In the present case, each nibble has 4 bits, which would normally permit 16 different nibbles, but nib ble 0000 is specified as an illegal nibble, so that NS = 15, making the Kraft equality: 120 N4E N8E-ME NI---N8E 152 When N4E is incremented by one (i.e., one more source character is added to the group being coded as one-nibble codewords), the equality requires NL-N8E to be incremented 65 by 15 and N8E-ME to be decremented by 14, and, conversely, if N4E is decremented by one, N L-N8 E must be decremented by 15 and NSE-ME must be incremented by 14. Therefore, if the source character of rank N4E occurs more often than the source characters of ranks N8E-1 5 through N8E-1, then N4E should be incremented by one (i. e., an additional source character should be added to the group being encoded as one-nibble codewords); and N4E should be decremented by one if the source character of rank ME-1 occurs less frequently than the source characters of ranks N8E to N8E + 14, because doing so will shorten the encoded stream.

The selection of N4E and N8E is implemented by counting from a middle value (e.g., 64) up for each appearance of the source characters of ranks N4E-1 and N4E and down for each appearance of the source characters of ranks N8E-1 5 to N8E + 14. When that count exceeds a first threshhold (e.g., 128), N4E is incremented; when the count fails below a second threshhold (e.g., 0), N4E is decremented. Once N4E is deter- mined, the value of N8E is recalculated in accordance with the Kraft equality.

The next step is to assign each source character to a codeword. The process is illustrated in Fig. 2, which represents a tree having a number of levels (columns) 110 equal to the largest number of nibbles which make up a codeword, in this case three. The first level 115 contains branches 112 equal in number to the possible number of nibbles (in this case 16) and each branch 112 of first level 115 represents either a possible 4-bit codeword or a possible 4-bit prefix to an 8-bit or 1 2-bit codeword. Each branch 112 is labeled with one number, not enclosed in par- entheses, indicating decimally the number of the nibble represented by that branch, and any branch 112 to which a source character may be assigned contains in parentheses the decimal rank of that source character. The second level 116 includes 16 sub-branches 118 for each branch 112 whose nibble is serving as a prefix. Each sub-branch 118 is similarly labeled with a nibble number, not in parentheses, and, in parentheses, the rank of the assigned source character. The third level 122 similarly includes 16 leaves 124 for each sub-branch 118 whose nibble was serving as a prefix. Each source character assigned to a leaf 124 at the third level would be encoded as a three-nibble word including the nibbles represented by the branch at the first level, the subbranch at the second level and the leaf at the third level.

Each branch, subbranch or leaf indicated as a dashed line represents the illegal nibble 0000 so that only 15 different nibbles are actually available at each level 110. The levels 110 of the tree of Fig. 2 are arranged to provide N4E available branches 112 at the first level for encoding N4E source characters 3 GB 2 152 252A 3 as one-nibble codewords, N8E-N4E available sub-branches 116 at the second level for encoding N8E-N4E source characters as twonibble codewords, and NL-N8E available leaves 124 at the third level for encoding the remaining source characters as three-nibble words, Encoder Structure As shown in Fig. 3, the encoder has input 75 16 on which it receives, possibly on an irregu lar schedule, the stream of 8-bit source char acters.

Memory M1 42 contains 256 elements representing the 256 possible source charac ters. The elements of memory M 1 42 are set to unique values between 0 and 255. The value r held in the ith element of memory M 1 42 represents the ranking r of the ith source character in frequency of appearance in the stream of source characters. For example, if the 1 Oth element of memory M 1 42 contains the value 128, then the source character represented by the 1 Oth element is estimated to be the 1 28th most frequently appearing character in the source stream.

Memory M2 44 contains 256 locations representing the 256 possible frequency rank ings of the source characters. The value i held in the rth element of M2 represents the 95 source character i which holds the rth rank in frequency of appearance. Accordingly, M1 is a table of rankings in source character order while M2 is a table of source characters in rank order. If the ith element of M 'I is r, the rth element of M2 is i. It is most convenient initially to set the ith elements of both M 'I and M2 to i.

Register N4E 46 contains the number N4E, and register N8E 48 the number N8E. N4E and N8E are initially set as non- negative integers such that the Kraft equality is satisfied, when NI- is chosen as 267, as follows:

15 N4E + N8E = 222 Register CE 64, which can initially be set to any value (e.g., 64), contains the count which forms the basis for changing N4E.

Arithmetic unit 60 performs required computations. The encoded stream of codewords is provided on output port 18. Register W 62 holds the nibbles of each codeword before transmission to port 18. Registers R 1 54, R2 56, A1 52, and A2 58 are connected as shown. Each component of encoder 14 is attached to data bus 49 and is connected to and controlled by data processor controller 50 as illustrated in Fig. 3. The components of encoder 14 are implemented as a conventional microprocessor and memory.

Encoder Operation The results of each numerical operation described below are rounded down to the nearest integer.

Referring to Figs. 3, 5A, 513, 5C and 51), when a source character byte is received on port 16, controller 50 moves it to register A1 52. The source character in register A1 52 then serves as an address pointer for table M 1 42, which provides the rank r of the source character to register R 1 54. If register R 1 54 does not then hold a 0 (i.e., the source character is not of the lowest rank, i.e., high est frequency of appearance), the rank value (r) is transferred from R '1 54 to register R2 56 which is then decremented by 1 to (r-1).

Register R2 56 then serves as an address pointer for table M2 44 and the source character having rank (r-1) is placed in register A2 58, which serves as an address pointer to register M1 44. The number in register A1 52 (i.e., the source character) is loaded into location (r-1) of table M2 44. Register R2 56 is then incremented by 1 to r and the number in register A1 52 is stored in location r of M2 44. Location A1 of table M '1 42 is decremented, and location A2 of table M '1 42 is incremented.

If the ith element of table M 1 42 is thought to contain the rank of source character i, and the rth element of table M2 44 is thought to contain the source character of rank r, the previous operation has the effect of placing the rank of the incoming source character into register R1 54, exchanging the rank of that source character with the rank of the next lower ranking source character, and updating tables M1 42 and M2 44 accordingly.

The contents of R l 54 (the rank r of the incoming source character) is compared with the contents of N4E 46 and N8E 48 and the value of CE is changed as follows. If r equals N4E or N4E-1, CE is incremented. If r is less than N8E + 15 and greater than or equal to N8E-1 5, CE is decremented.

Based on the results of the following comparisons between the contents of register R1 54, N4E 46 and N8E 48, the following actions are taken:

If R 1 is less than N4E (indicating that the incoming source character is within the group to be coded as one-nibble words), R '1 54 is incremented by 1 and transferred to register W 62, (i.e., that source character is coded as a codeword equal in number to one more than its rank-this is illustrated in Fig. 2 in which N4E = 10 and the source character of rank 0 is coded as nibble 1, while the source character of rank 1 is coded as nibble 2).

If R 'I is at least as large as N4E but less than N8E (i.e., the incoming source character is to be coded as a two-nibble word), then (N4E)l 5 + 17 + (NSE-N4E-1) is added to;,-,jister R 1 54 and transferred to 4 GB 2 152 252A 4 W62 (to become the potential two-nibble codeword). If the 4 least significant bits of the result are 0 (i.e., represent the illegal nibble (0000)), the value W + 15(N4E) + 15 16 is computed and the answer is transferred to 75 register W 62 (to become the final two-nibble codeword).

If R1 is at least as large as N8E (i.e., the codeword will be three nibbles long) 3829 is added to register R1 54 and the result placed in register W 62. If the 4 least significant bits of the result are not 0 (i.e., the third nibble is not illegal), the contents of register W 62 is divided by 16 and the result added to 3573 + N8E. If that result is less than 3856, 85 16 is subtracted from it. The result is sent to register W 62.

By the previous encoding operation, one, two or three non-zero nibbles are loaded into register W 62 as the codeword depending on the rank of the incoming source word. The zero nibble is not permanently written into register W 62 in a less significant position than the position of a non-zero nibble. The codeword is then transmitted from output port 18 beginning with the most significant non zero nibble.

Finally, if CE has reached the UPPER or LOWER threshhold value, CE must be reset to MIDDLE and N4E and N8E must be changed 100 to adjust the number of source characters being encoded as one-nibble, two-nibble and three-nibble codewords in order to minimize the length of the encoded stream. This is accomplished as follows.

If CE = UPPER and N8E is greater than 15, then CE is reset to MIDDLE, N4E is incremented by 1 and N8E is decremented by 15.

If CE = LOWER and N4E is not zero, CE is reset to MIDDLE, N4E is decremented by 1 and N8E is incremented by 15.

UPPER is the upper threshhold previously described and LOWER is the lower threshhold previously described. UPPER is greater than MIDDLE which is greater than LOWER, and these values control the rapidity with which the coding is changed as the relative frequencies of appearance of the characters in the source stream change. Values of UP- PER = 128, MIDDLE = 64 and LOWER = 0 are used for 8-bit source characters.

When CE grows large, N4E is increased causing more source characters to be encoded as single nibbles, and conversely, when CE grows small. The effect is to minimize the length of the stream of codewords as the relative frequency of appearance of the source characters shifts.

At this point, encoder 14 waits for the next source character to appear and the process is repeated.

After each source character is processed, encoder 14 contains tables (M1 and M2) reflecting the frequency ranks of all possible source characters, numbers (N4E and N8E) which determine the groups of source characters which will be encoded as one, two and three-nibble codewords, a number CE which measures the relative desirability of changing N4E and N8E, and a program for uniquely encoding the next received source character in accordance with its rank and the values of N4E and N8E.

Decoder Structure As shown in Fig. 4, decoder 24 has input port 22, on which it receives a stream of codewords, each comprising one, two or three 4-bit nibbles, output port 26 to which it delivers the stream of decoded source characters, and arithmetic unit 78 for performing computations. Table M3 82 corresponds to table M2 of the encoder, registers N4D 76 and N8D 66 correspond to registers N4E and N8E of the encoder, register CD 80 corresponds to register CE of the encoder and register S 74 holds a value which indicates whether the first, second or third nibble of a codeword is being processed. The compo- nents of decoder 24 are attached to data bus 79, and are controlled by data processor controller 70. The components of decoder 24 are implemented as a conventional microprocessor and memory.

Decoder Operation Prior to operation, table M3 is initialized to be the same as table M2 of the encoder, and register N4D, N8D and CD are set to the same values as the corresponding registers of the encoder. Register S is initially set to zero in preparation for processing the first nibble of the first incoming codeword.

Referring to Figs. 4, 6A, 6B, 6C, and 6D, when a nibble is received on port 22, controller 70 transfers it to register R3 72 and takes the following actions, depending on the value stored in register's 74. The value in register S indicates whether a first nibble, second nibble or third nibble of a codeword is being processed.

If S is zero, indicating the first nibble in a codeword, then register R3 is compared with the value in register N4D 76. If R3 is less than or equal to N4D (i.e., the codeword is only one nibble long) then R3 is decremented to obtain the rank of the source character; otherwise R3 is transferred to register S.

If S is greater than 0 and less than 16, indicating the second nibble in a codeword, the content of register S 74 is multiplied by 16 and the result is added to R3. If R3 is less than 16 N4D + 17 + (N81)-N4D), then 15 N4D + 15 is subtracted from R3, the result is multiplied by 16 and returned to R3. If R3 is then less than 15 N4D + 17 + (N81)-N4D) + NSD 75 then N4D + 17 + (N81)-N4D) is subtracted from R3 and S is cleared, other- 85 wise R3 is sent to S.

If S is greater than or equal to 16, indicat ing the third nibble of a codeword, then the content of register S 74 is multiplied by 16 and the result is added to R3. If R3 is less than 3840, then 16 is added to it and, if the resulting R3 is less than 3829 + N8D, then 3573 + N8D is subtracted from R3, the result is multiplied by 16 and that result is stored in R3. Then 3829 is subtracted from R3 and S is cleared.

When these operations are completed, if the value of S is 0, indicating that a new codeword is about to be received, the value in CD must be updated to conform to the value 100 of CE so that the encoder and decoder will be operating with the same codes. This is accom plished by controller 73 performing the fol lowing functions:

The contents of R3 (which contains the rank of the source character corresponding to the codeword just decoded) are compared with the contents of N4D and N4E. If R3 = N4D-1 or R3 = N4D, then register CD 80 is incremented. If R3 is greater than or equal to N81)- 15 and less than N8D + 15 then register CD 80 is decremented.

Controller 70 then places in output port 26, the contents of location R3 of table M3 82 (which is the source character of rank R3) 115 and, if R3 is greater than zero (that is the source character is not of the lowest rank), the contents of locations R3 and R3-1 in table M3 are interchanged, which effectively ex changes the ranks of the source character just 120 processed and the source character of the next lower rank, thereby causing table M3 to correspond to table M2 44 of encoder 14.

Next, controller 70 checks the value of CD against the same threshholds, UPPER and 125 LOWER, used by encoder 14 and performs the following:

If CD = UPPER and N8D greater than 15, then CD is set to MIDDLE, N4D is incre mented by 1 and N8D is decremented by 15. 130 GB 2 152 252A 5 If CD = LOWER and N4D is greater than 0, then CD is set to MIDDLE, N4D is decremented by 1 and N8D is incremented by 15.

These steps have the effect of internally updating the values of CD, N4D and N8D to correspond exactly to CE, N4E and N8E, respectively, in encoder 14.

Finally, the decoded source character on output port 26 is provided to line 28. Controller 70 waits until the next code word appears on input port 22 and then repeats the process.

Decoder 24 therefore decodes each received codeword into the corresponding source character using the reverse process of the encoder based on decoding parameters N4D and N8D and a list M3 of source characters in rank order. N4D and N8D are changed when a value CD exceeds preprogrammed thresholds, UPPER and LOWER which are the same as those programmed into encoder 14. CD is updated to be the same as value CE. M3 is updated to show the current rankings of the source characters by the name process used in encoder 14. No information other than the source character stream needs to be transmitted from encoder 14 to decoder 24 to coordinate their respective activities, since the required coding information is implicitly con- tained in the codewords themselves.

Further variations are possible. For example, the codewords can include a group of codewords made up of more than three nibbles. Then, the dividing points between the groups of codewords are determined by more than two partitioning values under the Kraft equality, and more than one of those values will be free variables with the remaining values being determined once those free variables are set.

Claims (4)

1. Apparatus for encoding a stream of source characters into a stream of codewords, each of said source characters having a time- varying frequency of appearance in said stream of source characters, said apparatus comprising: a rank store for storing the rank corresponding to the current estimated relative frequency of appearance of each said source character in said stream of source characters; and a rank updater for exchanging the rank of each said source character which appears in said stream of source characters with the rank of the next lower ranking one of said source characters, unless said source character which appears in said stream has the lowest possible rank.

2. Apparatus according to Claim 1, and including means for encoding according to a selectable code each of said source characters having a rank not higher than a selectable first number as a codeword having one subword, each of said source characters having a rank higher than said first number and not higher than a second number as a codeword having 6 GB2152252A 6 two subwords, and each of said source characters having a rank higher than said second number as a codeword having three subwords, said second number being fixed by 5 the value of said first number.

3. Apparatus according to Claim 2, further comprising a decoder for decoding said stream of codewords into said stream of source characters, said decoder comprising: a decoder frequency ranker for determining for each of said source characters in said source alphabet said rank corresponding to its said current estimated relative frequency of appearance; said decoder being adapted to deter- mine directly from said codewords in said stream of codewords which one of said selectable codes was used for said encoding.

4. Apparatus according to Claim 1 and substantially as hereinbefore described with reference to and as shown in the accompanying drawings, for encoding a stream of source characters into a stream of codewords.

Printed in the United Kingdom for Her Majesty's Stationery Office. Dd 8818935. 1985. 4235. Published at The Patent Office, 25 Southampton Buildings. London, WC2A 'I AY, from which copies may be obtained.